Various methods for producing lithium hexafluorophosphate, which is an electrolyte useful for lithium ion batteries and the like, have been proposed. As a normal electrolyte solution production, a method is conducted in which firstly lithium hexafluorophosphate is produced, and it is dissolved in a predetermined lithium battery solvent to make an electrolyte solution. Regarding the method for producing lithium hexafluorophosphate, for example, there is a method (Patent Publication 1) in which lithium fluoride in solid and phosphorus pentafluoride in gas are reacted with no solvent. In this method, there is a danger that a film of the reaction product is formed on the surface of lithium fluoride, thereby the reaction does not proceed completely, and the unreacted lithium fluoride remains. There is also a method (Patent Publication 2) in which a reaction is conducted by adding anhydrous hydrogen fluoride to phosphorus pentachloride and lithium fluoride with no solvent as above. The reaction control is difficult in this, and cooling down to several ten degrees below freezing point is necessary.
On the other hand, in the methods for producing lithium hexafluorophosphate using solvent, there is a method (Patent Publication 5) in which gaseous phosphorus pentafluoride is reacted with lithium fluoride dissolved by using anhydrous hydrogen fluoride as solvent, and the resulting lithium hexafluorophosphate is crystallized and taken out.
In this method, conversion of lithium hexafluorophosphate is high, but it is necessary to use a large amount of anhydrous hydrogen fluoride, which is high in vapor pressure and has toxicity and corrosiveness, as solvent. Therefore, handling is not easy. Furthermore, there are many factors related to high cost, such as necessity of producing phosphorus pentafluoride, which is one of the raw materials, by another process, and necessity of providing a process for crystallizing lithium hexafluorophosphate.
There is a method (Patent Publication 3) in which lithium fluoride and phosphorus pentafluoride are reacted in organic solvent. This method has a great advantage in terms of reaction control and purity, but as mentioned above it is necessary to produce and handle phosphorus pentafluoride gas, which is one of the raw materials, by another process. Therefore, a problem of cost remains.
Furthermore, there is also a method (Patent Publication 4) in which phosphorus trichloride, chlorine and hydrogen fluoride are reacted by using anhydrous hydrogen fluoride or CH3CN, which is a polar organic solvent, as the solvent to obtain phosphorus pentafluoride, followed by adding lithium fluoride to the same reactor to react it with phosphorus pentafluoride, thereby producing lithium hexafluorophosphate.
This method is efficient since the production of phosphorus pentafluoride is also conducted in the same reactor. However, it goes through the production of phosphorus pentafluoride, which is high in vapor pressure. Therefore, it is necessary to have expensive facilities such as pressurized reactor and complicated operations. Basically, the crystallization process is necessary. Therefore, many problems remain such as difficulty of drastic cost reduction in the electrolyte solution production.    Patent Publication 1: Japanese Patent Publication 64-72901    Patent Publication 2: Japanese Patent Application Publication 10-72207    Patent Publication 3: Japanese Patent Application Publication 9-165210    Patent Publication 4: Japanese Patent Application Publication 10-81505    Patent Publication 5: Japanese Patent Application Publication 6-56413